Background on Coal Plant Conversions to Natural Gas

There is a growing trend in the utility industry to convert existing coal-fired power plants to burn what are considered to be more environmentally-friendly fuel types, such as biomass and natural gas, though whether such conversions are environmentally beneficially remains controversial. This trend is driven by a number of factors, including state-level renewable portfolio standards; federal incentives and looming environmental regulations; consumer demand and environmental awareness; and an economic climate that is making coal less attractive.[1] Although conversion costs can be expensive, utilities already have the facilities sited and water supply and transmission lines established. Converting existing facilities can often cost less than installing the emissions control systems required to keep an antiquated coal plant running.[2]

Some coal-fired power plants have been converted to burn natural gas, the environmental impacts of which are better understood than those of biomass. Natural gas combustion produces almost 45 percent fewer carbon dioxide emissions than coal, emits lower levels of nitrogen oxides and particulates, and produces virtually no sulfur dioxide and mercury emissions. The lower levels of these emissions mean that the use of natural gas does not contribute significantly to smog or acid rain formation. In addition, because natural gas boilers do not need the scrubbers required by coal-fired power plants to reduce SO2 emissions, natural gas plants create much less toxic sludge. [3]

However, natural gas is still a fossil fuel and the potential for leakage in pipes has been increasing drawing attention from anti-global warming advocates.[4] Although the carbon content of natural gas is lower than that of coal, it nonetheless releases harmful CO2 into the atmosphere when burned.[5] Its extraction from shale, the most significant new source of natural gas, can have harmful impacts on water, land use, and wildlife, if the process is not managed properly. As with biofuels, many enviromentalists do not see natural gas as a longterm solution for the nation's fuel needs. In July 2009, Robert F. Kennedy Jr. published a column acknowledging the "environmental caveats" that come with converting coal plants to natural gas. He and other environmental advocates, however, do support natural gas as a short-term solution to reduce the environmental burden of coal until renewable solar, wind, and geothermal technologies can be implemented to their full potential.[6]

Problems with Leaking Natural Gas Mains

Natural gas is composed primarily of methane. When gas mains carrying natural gas leak, methane is released into the atmosphere. Methane (CH4) is a greenhouse gas and plays a significant role in the dynamics of global warming due to the following factors:

Methane is a relatively potent greenhouse gas with a high global warming potential 72 times that of carbon dioxide (averaged over 20 years) or 25 times that of carbon dioxide (averaged over 100 years), according to the IPCC's Third Assessment Report.[7] (Note that the global warming potential of methane was estimated at 21 times that of carbon dioxide, averaged over 100 years, in the IPCC Second Assessment Report, and the 21 figure is currently used for regulatory purposes in the United States.[8]) Methane in the atmosphere is eventually oxidized, producing carbon dioxide and water. This breakdown accounts for the decline in the global warming potential of methane over longer periods of time.

According to calculations reported in 2005, methane emissions may account for a third of the climate warming from greenhouse gases between the 1750s and the present, twice the level of previous estimates.[10]

An average molecule of CH4 lasts around eight to nine years before it gets oxidized into carbon dioxide (CO2) and water (H2O).[9]

Leakage Rates Worldwide and in the United States

Worldwide leakage rates, however, may vary significantly. The major source of natural gas losses from local distribution systems is cast iron distribution pipes. Recent data on leakage rates from cast iron distribution pipelines highlights the potential variability of actual leakage rates around the world. Most of the pipes installed prior to the 1950s were cast iron.[11]

In 1990 the Earth Resources Research, an environmental consulting firm based in the United Kingdom, released a report funded by Greenpeace which indicated that natural gas pipes in the country leaked so much methane that the leaking alone contributed more to the greenhouse effect than the burning of the fossil fuel.[12] According to report authors, Catherine Mitchell and Jim Sweet of Earth Resources Research, a substantial increase in the use of natural gas could well increase the greenhouse effect, not decrease it. As noted, natural gas is composed mainly of methane, itself a greenhouse gas some 60 times as powerful, over 20 years after it is released, as carbon dioxide.

If natural gas mains leak enough methane, the Greenpeace study reports, this could mean any advantage gained by generating less carbon dioxide through switching to natural gas could be undermined. The authors of the study estimated 5.3 to 10.8 percent of the gas flowing through Britain's natural gas pipes are leaking each year. Using United Nations Intergovernmental Panel on Climate Change figures, the researches estimated that leaked methane would have more impact on warming over 20 years than the carbon dioxide produced when the gas is burnt. They calculated that a leakage of 2.8 percent would cancel any greenhouse advantage of gas over fossil fuels like oil and coal.[12]

Leakage Rates in the United States

In the United States, methane emissions from natural gas distribution mains accounts for 32 percent of the industry's total methane emissions. It is believed that cast iron pipelines contribute the most to these emissions, even though they represent only 3% of the miles of all U.S. distribution mains. These estimates are based on national methane leak rates from an EPA-funded study which estimated emissions from all sources in the U.S. natural gas industry. [13]

Since 1992 the EPA has gathered over 100 companies to participate in their Natural Gas Star Program, which aims to reduce the amount of methane leakage from distribution pipe systems. It is a voluntary program and not required. In 1997, as a result of the Star Program, the U.S. Environmental Protection Agency EPA released a report which indicated that a potential increase in natural gas sales would likely increase methane output by 0.5 to 1 percent annually. Using 1992 as their baseline, the EPA estimated that approximately 1.4 percent (plus or minus .5 percent) of all gas that travels through pipes in the United States was emitted. Overall, of all the methane released by industry in the United States, 20 percent of methane comes from the natural gas sector. Landfills contribute the most with 31 percent.[14]

In the same report, the EPA stated that of the methane released by the natural gas industry, 37 percent comes from "Transmission/Storage", 24 percent comes from "Distribution" and 27% during production. The EPA noted that during summer peak times, emissions were estimated to the highest. The study, contrary to the more recent findings by a Greenpeace funded study in Europe, argues that, using estimated emissions from 1992, that the natural gas sector emits less greenhouse gas emissions than coal or oil.[14]

Currently it is estimated that 2% of total greenhouse gas emissions come from the country's natural gas industry. In 2006, the natural gas industry operated over 38,000 miles of natural gas pipelines that were made of cast iron, the leakiest of all types of gas piping. In 2009, 4,000 miles of new pipes were laid.[15] Further studies of methane gas loss rates need to be completed to access the situation globally. Assessing these loss rates will help reduce methane leaks from natural gas distribution in the United States.[16]

As of 2010 Russia and the United States are the leading sources of natural gas leaks in the entire world, according to the Environmental Protection Agency’s 2009 official estimate. The New York Times reports that, "This amount has the warming power of emissions from over half the coal plants in the United States." Government scientists and some industry officials caution that the real figure is actually higher. These officials suggest that monitoring of natural gas leaks in pipes and storage facilities, needs to be greatly expanded. Fugitive emissions could soar as global production of natural gas increases over the next few decades. Production is estimated to rise almost 50 percent in the next 20 years.[15]

In an April 2013 report the EPA lowered its estimate of how much of a potent heat-trapping gas leaks during natural gas production. It was reported that "tighter pollution controls instituted by the industry resulted in an average annual decrease of 41.6 million metric tons of methane emissions from 1990 through 2010, or more than 850 million metric tons overall. That's about a 20 percent reduction from previous estimates. The agency converts the methane emissions into their equivalent in carbon dioxide, following standard scientific practice."[19]